It is known that in systems employing computers or the like, it is economic to have the computer perform as many tasks as possible. These tasks may include generation of signals employed in telephone signaling, for example, multifrequency signals including ringback and the like.
Multifrequency signals have heretofore been generated by employing a plurality of digital signal generators and associated complex filters to obtain desired analog signals.
More recently, similar multifrequency signals have been generated by storing digital samples representing amplitude values of desired tones of the multifrequency signal and sampling those samples at a frequency greater than the theoretical Nyquist rate (at least twice the frequency of the highest frequency tone in the multifrequency signal) in order to obtain a desirable multifrequency signal. One example of an arrangement which employs a plurality of read only memories (ROMs) for storing signal samples to generate a multifrequency signal is disclosed in U.S. Pat. No. 4,171,466 issued to Robert L. Carbrey on Oct. 16, 1979.
Similarly, multifrequency signals have been generated by appropriately programming a so-called microcomputer system. Specifically, the microcomputer is programmed to switch a plurality of internal timers on and off at intervals corresponding to square wave signals. The square waves are combined and filtered to obtain a desired analog multifrequency signal.
Computers have also been employed to generate digital signals at predetermined frequencies by sampling stored digital representations of amplitude values of the desired signals at a sampling rate significantly higher than the Nyquist rate. Use of a "high" sampling rate eliminates so-called "alias" signals from being at frequencies which would be difficult to filter.
All of the prior known multifrequency signal generators have limitations.
Specifically, use of a plurality of generators, digital or otherwise, is undesirable from cost and size considerations. When using digital signal generators, a relatively complex filter is required to eliminate undesirable harmonic frequency components, especially the third harmonic.
Use of a plurality of read only memories and associated circuit arrangements to generate a multifrequency signal as disclosed in U.S. Pat. No. 4,171,466 is also undesirable in certain applications because of the number of circuit components involved. Additionally, it also appears that the sampling rate employed is above the Nyquist rate. Moreover, each sample must be sampled a plurality of times. Although satisfactory for certain applications, this would be undesirable for others because of the additional time required, especially when the number of tasks to be performed limits the time available for generating the multifrequency signal.
When using a computer to generate a number of square waves corresponding to the number of tones in a multifrequency signal, a complex filter is required to obtain a so-called "clean" signal. More importantly, as the frequencies of the tones become closer, the switching speed of the computer must be faster. Consequently, for signals having tones at closely spaced frequencies, a high quality, high speed and, hence, more expensive computer is required. This imposes a severe economic burden on many systems.
Similarly, use of a computer to generate desired signals by sampling stored digital representations of amplitude values using as high a sampling rate as possible to eliminate harmonics requires a large number of samples to be stored and also requires a high speed computer. The additional memory required to store the samples and use of a high speed computer are not economically justifiable in many applications.